A direct injection engine having an injector for injecting fuel directly into a combustion chamber is conventionally known. This engine is operated such that a condition in which a mixture is locally distributed around a spark plug is produced by injecting the fuel in a latter half of a compression stroke to perform so-called stratified charge combustion in a low-load state. This operation makes it possible to increase the air-fuel ratio (leaner mixture) while maintaining combustion stability and improve fuel economy.
Exhaust gases from engines of motor vehicles, for instance, contain hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx), and there exists a demand today to reduce generation and release of these harmful constituents as much as possible to achieve improved properties of these emissions. One approach that has conventionally been taken is to provide a catalyst in an exhaust passage, and it is a common practice in the aforementioned direct injection engine as well to provide a catalyst in its exhaust passage. A generally known example of such catalyst is a three-way catalyst which has the ability to clean out HC, CO and NOx approximately at the stoichiometric air-fuel ratio. Another example that has already been developed is a catalyst which can clean out NOx even in a "lean" operating range in order to be suited to lean burn operation by stratified charge combustion in the aforementioned direct injection engine or else.
A fuel injection control device disclosed in Japanese Unexamined Patent Publication No. 4-231645, for example, is known as a device for achieving an improvement in conversion efficiency of a catalyst at low temperatures, for instance, in this type of direct injection engine. In a direct injection engine having a lean NOx catalyst provided in an exhaust passage, the lean NOx catalyst being of a type that requires HC for the reduction of NOx, this device is so arranged as to make primary injection from an injector in a latter part of a compression stroke, and make secondary injection in addition to the aforementioned primary injection to inject a small amount of fuel for supplying HC to the lean NOx catalyst within a period from an intake stroke to an early part of the compression stroke when the temperature of the catalyst is low, or make the aforementioned secondary injection in addition to the aforementioned primary injection within a period from a latter half of a combustion stroke to an early part of an exhaust stroke when the temperature of the catalyst is high. In this device, HC derived from the fuel injected by the secondary injection is supplied to the catalyst in the exhaust passage by setting the amount of fuel injected by the secondary injection to such a small level that will scarcely affect combustion within a combustion chamber, and a low boiling-point constituent of EC is supplied to the catalyst in low-temperature conditions and a high boiling-point constituent of HC is supplied to the catalyst in high-temperature conditions by varying the timing of secondary injection in low-temperature and high-temperature conditions in the aforementioned manner.
A catalyst for exhaust gas conversion can not fully exhibit its conversion effects when the catalyst is not heated yet and the catalyst temperature is lower than its activation temperature. HC and NOx are likely to be released in large quantities in such a case. For this reason, it is required to reduce the amounts of HC and NOx emissions output from the engine to the exhaust passage and to promote catalyst quick light-off operation by increasing the exhaust gas temperature while the catalyst is still in its unheated state, for instance.
The aforementioned direct injection engine, however, has a problem that its catalyst is not easily heated because the amount of heat released into the exhaust passage is small in a case where the air-fuel ratio is increased by performing stratified charge combustion through injection in the compression stroke even when the catalyst is not heated yet.
Although the device disclosed in the aforementioned Publication is intended to achieve an improvement in low-temperature catalyst performance by making, in addition to the primary injection during the latter part of the compression stroke, the secondary injection preceding the primary injection when the temperature of the catalyst is low, the amount of fuel injected by this secondary injection is extremely small and is delivered to the catalyst in the exhaust passage almost without burning within the combustion chamber. Therefore, this device is advantageous only when a lean NOx catalyst of a type that requires HC for the reduction of NOx is used. Moreover, the device makes it possible to achieve NOx conversion effects with the supply of HC only after the catalyst has been activated to a certain degree, though it is still in a low-temperature state, and because HC is released in an earlier unheated state than that point, the device is not favorably suited for achieving an improvement in emissions. Furthermore, the device does not have the function of promoting the catalyst quick light-off operation by an increase in the exhaust gas temperature.
Another measure to deal with cold start of a direct injection engine is, as disclosed in Japanese Unexamined Patent Publication No. 4-187841, such that ignitability is maintained by increasing the amount of fuel injected during the compression stroke while the internal temperature of cylinders is low. More specifically, the engine is controlled to make injection during the compression stroke in a low-load range, split injection during the successive intake and compression strokes in a medium-load range and injection during the intake stroke in a high-load range when the engine is in its warm-running condition, whereas the aforementioned range of split injection is extended to the high-load side while the engine is still cold.
This device, however, maintains the ignitability simply by increasing the amount of fuel injected in the compression stroke by as much as an amount corresponding to deterioration of evaporation and atomization while the engine is cold, and the device does not have the ability to accomplish quick light-off and emission improvement by an increase in the exhaust gas temperature while the catalyst is still in its unheated state.
In the light of the above-described circumstances, it is an object of this invention to provide a direct injection engine capable of reducing the amounts of emissions, such as HC and NOx, from the engine and promoting light-off of a catalyst by increasing the exhaust gas temperature while the catalyst is still in its unheated state, for instance, so that the emissions are significantly achieved through a reduction in the time required until the catalyst is brought to its unheated state and a reduction in the amounts of HC and other emissions when the catalyst is not heated yet.